Sorry to break the trend but I definitely prefer May 31. June 7 is DC Pride weekend which means the Meridian Hill region of town is going to be packed with all kinds of craziness. Awesome craziness that is, but still, I'd go hands down the 31st so we don't have to fight parades and crowds and conflicting invites.
And also, for the record, most of the DC people who participated in the last meetup (myself included) seem to have moved to the Google Groups for coordination, I'd definitely recommend either starting these discussions there in the future or, at the least, keeping that forum up to date. I think you'll see better responses doing that. Besides, we'd love to have any other DCers from this forum join our discussions there anyway!